Analysis: Anthony Zurcher, BBC News, Houston, Texas

He can also serve as an attack dog, tearing into Marco Rubio the way he did during that fateful debate in New Hampshire.

The Christie endorsement could signal a new phase of Mr Trump's presidential campaign, when prominent politicians begin to make peace with the reality of a Trump nomination.

Mr Christie, and others, may see a benefit to being among the first major figures on board the Trump bandwagon. Already rumours are floating that former candidate Mike Huckabee is on the verge of joining Mr Trump's ranks as well.

This is what happens when a frontrunner draws close to victory. And just because the man approaching the finish line is Donald Trump doesn't make it any less true.

Frontrunner Donald Trump, a businessman from New York, is leading in many state polls and has already won three consecutive state contests in New Hampshire, Nevada and South Carolina, despite never having worked in politics.

He shouldered some criticism from Mr Rubio and Mr Cruz at a Republican debate on Thursday but it is not yet clear whether this has hurt his popularity.

"He is rewriting the playbook of American politics because he's providing strong leadership that is not dependent upon the status quo," Mr Christie said of Mr Trump.

"I will lend my support between now and November in every way that I can for Donald, to help to make this campaign an even better campaign than it's already been."

Mr Rubio, the Florida Senator, continued to assail Mr Trump the morning after the debate.

He told CBS: "A con artist is about to take over the Republican Party and the conservative movement, and we have to put a stop to it."

And at a morning rally in Dallas, he mocked the Republican front-runner about his misspelled tweets and said he had seen Mr Trump having a "meltdown" backstage at the debate.

Republican race: Who's who?

Donald Trump: Billionaire businessman and son of New York real estate tycoon Fred Trump. Now a frontrunner for the Republican nomination after attracting previously disaffected voters with controversial, populist platform

Chris Christie: Governor of New Jersey who until this month was running for the nomination himself. He has been thought of as being from a more moderate wing of the party and had called Mr Trump's proposed ban on Muslims entering the US "ridiculous"

Marco Rubio: Florida senator and son of Cuban immigrants. Seen as the Republican establishment's challenger to Mr Trump, but has struggled to effectively counter Mr Trump's forthright style

Ted Cruz: Texas senator, favoured by evangelicals and many in the conservative Tea Party faction, has also sold himself as an "anti-establishment" candidate